I never use the default directories, but in theory they are still vulnerable to ransomware attack. But then again I expect Avast to protect against ransomware attack, after all it is an antivirus/malware product, or are they saying that they can't.
If you are an Avast Free user, the smart scan in my humble opinion is more of a sales opportunity than any real protection. Which is one reason I rarely run any on-demand scan (outside of beta testing and questions in the forums) and this include the so called smart (NOT) scan.